This application is a 371 National stage filing of international application PCT/EP00/11083 filed Nov. 9, 2000.
The invention relates to a method for modifying a wooden surface, an electrode being arranged opposite to the wooden surface to be modified, and an alternating high voltage being applied to the electrode, in order to cause a discharge between the electrode and the wooden surface under atmospheric pressure.
A method of the kind described at the beginning is known from U.S. Pat. No. 5,215,637. This document focuses on enhancing the bond properties of plastic surfaces with regard to adhesives, paints, coatings and the like. However, it is also mention, that other objects having a high dielectric constant such as ceramics, cardboard, paper and wood can be treated in the same way. In the known method an object having the surface to be modified is arranged between a pair of electrode plates to which reverse alternating high voltages of 125.000 Volt are applied, the difference voltage falling over an air gap of about 40 cm between the electrode plates. The frequency of the alternating high voltage is 60 Hz. By means of the applied alternating high voltages a corona-discharge is cost at atmospheric pressure which acts upon the surface to be modified. The corona discharge has a gore structure of the conducting plasma within the cross section of the air gap. To evenly distribute the conductive plasma in the corona-discharge over the cross section of the air gap between the electrode plates, the electrode plates are provided with perforated polyethylene shields. The perforations then each correspond to a discharge path between both electrode plates. Nevertheless in the known method there is a comparatively uneven modification of the surface to be modified by the effects of the plasma. This is particularly the case, if in deed a wooden surface is treated with the known method as wood typically has an inhomogen structure so that its dielectric properties are not evenly distributed over the wooden surface to be modified. As the result this means that some areas of the wooden surface are subject to a much stronger modification as other regions.
A method for modifying a wooden surface in which the wooden surface is removed layer by layer is known from DE 197 18 287 C1. Here, the surface to be modified is locally heated up by energy rich radiation so that a thin wood layer is suddenly heated up to such an extend that the essential part of the heated up volume is also suddenly evaporated and transformed into plasma. As energy-rich radiation particularly laser radiation may be taken into a count. In treating larger wooden surfaces the affords with regard to the apparatus for carrying out the known method are however very high. Because of principal reasons only a very small part of the wooden surface can be treated at each time to avoid an undesired heating up of the entire work piece.
This it is the problem of the invention to provide a method of the kind described at the beginning by which the wooden surface to be modified can be modified more evenly, and which, at the same time, can be carried out with large wooden surfaces within acceptable intervals of time at acceptable cost.
According to the invention this problem is solved in that a dielectric layer is arranged between the electrode and the wooden surface to be modified and in that the alternating high voltage is applied with a frequency of more than 600 Hz.
The first feature of the new method results in a dielectric hindered discharge being caused which acts upon the wooden surface to be modified. As compared to a corona-discharge, a dielectric hindered discharge is principally indicated by a much finer distribution of the plasma, i.e. of the actual discharge areas over the whole cross section covered by the discharge. The alternating high voltage having a frequency of more then 600 Hz also contributes to the fine distribution of the plasma. All at all the desired homogenous modification of the wooden surface is achieved. At the same time the cost of carrying out the new method are limited. The energy input is in the order of 1 kWh per mo2 wooden surface, and can thus be estimated as comparatively low.
The new method is not only applicable to enhance the bond of different coatings to the wooden surface, which already includes coating with enhancive and thus gluing together via the wooden surface. The modification of the wooden surface can also be conducted as a upgrading step for the wooden surface which is not followed by a coating of the wooden surface. This includes, for example, removing loose or damaged parts of the wooden surface, which is desirable after sawing or during restoration of wooden work pieces. Further, also a preservation of the wooden surface can be effected by the method according to the invention.
To achieve the respective desired effects by modifying the wooden surface, the atmosphere in which the discharge between the wooden surface and the electrode is caused may be modified with regard to normal air by adding certain gases. This is particularly valid, if this gases are to be intercalated into the wooden surface to be modified. The gas mixtures desired in each particular case can simply be blown into the zone of the discharge as it takes place under atmospheric pressure. A thermal stress of the wooden surface does not take place during the new method. The gas temperature in the area of the discharge does not essentially rise above room temperature.
In the new method, a piece of wood having the wooden surface to be modified can be connected to the ground as the counter electrode for the electrode. I.e. for carrying out the new method only a single further electrode is necessary besides the piece of wood having the wooden surface to be modified. The conductive properties of wood are sufficient for forming the counter electrode.
A piece of wood having the wooden surface to be modified can however, also arranged on a plan counter electrode arranged in parallel to the plan electrode. In this case the piece of wood has the effect of a second dielectric layer in front of the counter-electrode. I.e., in this case the dielectric properties of the wood are dominant is compared to its conductivity.
In a preferred embodiment of the new method a piece of wood having the wood surface to be modified is moved on a conveyor belt made of dielectric material over a plane counter-electrode arranged in parallel to the plane electrode. Here, both electrodes, i.e. the electrode facing the wooden surface to be modified in the counter-electrode stands still and the piece of wood is transported there between. Here, the conveyor belt for the piece of wood at the same time surfs at a dielectric layer in front of the counter-electrode.
In modifying very large wooden surfaces to be modified it is suitable to move the electrode with regard to a piece of wood having the wooden surface to be modified, i.e. in parallel to the wooden surface to be modified. Moving the electrode with regard to the piece of wood can either be accomplished by moving the piece of wood having the wooden surface to be modified or the electrode itself.
If the surface of the electrode facing the wooden surface to be modified is small as compared to the surface of the wooden surface to be modified, even strongly contoured wooden surfaces can be scanned under defined discharge conditions to accomplish the desired modification of the wooden surface.
Preferably the surface of the electrode as compared to the surface of the wooden surface to be modified is dimensioned in the new method in such a way that the wooden surface to be modified covers at least 90% of the cross section of the discharge. With other words, the discharge power is used in the new method as far as possible for the desired modification of the wooden surface. As little discharge power as possible shall be consumed besides the wooden surface to be modified, i.e. directly between the electrode a counter-electrode.
To achieve a particularly good distribution of the plasma of the discharge over the wooden surface to be modified, the alternating high voltage is preferable applied with a frequency of over 5 kHz, i.e. preferably of 10 to 3000 kHz, in the new method.
Here, it is particularly preferred, if the alternative high voltage is consisting of single high voltage pulses, the distance of which is greater then their duration. For example, the distance of the single high voltage pulses can be correspond to a frequency in the range of 10 to 20 kHz, whereas the duration of the single high voltage pulses may comprise frequency components in the range of over 500 kHz.
If, in the new method, the alternating high voltage is applied to the electrode with alternating polarity, the building up of charges both at the dielectric layer in front of the electrode and at the wood surface to be modified is avoided.
The distance of the electrode with the dielectric layer from the wooden surface to be modified is typically between 1 and 25 mm in the new method. I.e., this distance is not critical. It is to be understood, however, that with increasing distance the alternating high voltage has to be raised. Typical values for the alternating high voltage are 30 to 50 kV with single high voltage pulses. In case of a sinusoidal high voltage, the frequency of which can also be in a range of above 100 kHz, for example between 100 and 3,000 kHz, the alternating high voltage is typically 10 to 15 kV.